The present inventions relate generally to a laboratory or portable measurement method and system, and more particularly, to a method and system for the point of sale measurement of the water and sediment content in a petroleum sample.
Conventional Methods for Measuring Water Content
A determination of water content in crude oil is required to measure accurately net volumes of actual oil in sales, taxation, exchanges, and custody transfers. The water content of crude oil is also significant because it can cause corrosion of equipment and problems in processing. Thus, various methods have been developed for measuring the water content of crude oil.
Karl Fischer Titration Method
In 1935, German scientist, Karl Fischer, developed a titrimetric determination of water content using a reagent that contained iodine, sulphur dioxide, anhydrous pyridine, and anhydrous methanol. This method can be subdivided into two main techniques: volumetric titration and coulometric titration.
The volumetric technique involves dissolving the sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. When all of the water has reacted, the platinum measuring indicator electrode will electronically instruct the burette to stop dispensing. The volume of KF reagent dispensed is recorded. Based on the concentration of iodine in the KF reagent, the amount of water present is then calculated.
However, even with the automatic or semi-automatic instruments commercially available, there are certain problems associated with this technique. These problems include long analysis time, required reagent calibration, and high reagent consumption rate.
In the coulometric technique developed by Meyer and Boyd in 1959, the sample is introduced into a mixture of pyridine/methanol that contains iodide ions and sulphur dioxide. The electrode system consists of an anode and cathode platinum electrodes that conduct electricity through the cell. Iodine is generated at the anode and reacts with any water present. The production of iodine is directly proportional to the amount of electricity according to Faraday's Law as shown in the equation:2I−I−2e→I2.
According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. Therefore, it is possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs. The platinum indicating electrode voltametrically senses the presence of water and continues to generate iodine until all the water in the sample has been reacted.
From this titration, the onboard microprocessor calculates the total amount of current consumed in completing the titration and the time to completion in seconds. Based on the relationship between coulombs and iodine, the exact amount of iodine generated is recorded.
Since water reacts in the 1:1 ratio with iodine, the amount of water can be calculated.
Although the original Karl Fischer reagent contained pyridine, most reagent manufacturers now use other amines such as imidazol.
Karl Fischer titration is one of the most widely used techniques for measuring the water content in a large range of samples. However, it has limits that affect its usefulness. For example, it utilizes hazardous reagents that require the operator to exercise care in the storing, handling, and disposing of reagents that degrade with time. With the techniques, a total sample size of 0.5 ml. or smaller is taken from a larger sample size, typically 250 ml. The small sample size utilized by the techniques causes errors and cannot measure water percentages over 1% accurately. Also, the Karl Fischer titration techniques are operator intensive and do not provide any information with regard to the amount of sedimentation in a sample.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.7—Standard Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration and Chapter 10.9—Determination of Water in Crude Oils Coulometric Karl Fischer Titration for the complete protocols, which are hereby incorporated by reference.)
Centrifuge Method
In the standard method for determining the water content in crude oil by centrifuge, equal volumes of a sample and water saturated toluene are placed into two cone-shaped centrifuge tubes. The tubes are then corked and placed into a centrifuge. The tubes are then spun, for example, for 10 minutes at a minimum relative centrifugal force of 600 calculated from the following equation:rmp=1335√{square root over (rcf/d)}
where:                rcf=relative centrifugal force and        d=diameter of swing measured between tips of opposite tubes when in rotating position, mm.        
Immediately after the centrifuge comes to rest following the spin, the combined volume of water and sediment at the bottom of each tube is read and recorded. The spin is then repeated until the combined volume of water and sediment remains constant for two consecutive spins. The final volume of water is then recorded for each tube.
The standard method for determining the water content by centrifuge is not entirely satisfactory. The amount of water detected is almost always lower than the actual water content. Therefore, when a high accurate value is required, another method must be used. This method also requires hazardous solvents and has very poor accuracy and reproducibility.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.3—Standard Test Method for Water and Sediment in Crude Oil by the Centrifuge Method (Laboratory Procedure) for the complete protocol, which is hereby incorporated by reference.)
Distillation Method
In the standard test for determining the water content by distillation, the sample is heated under reflux conditions with a water immiscible solvent that co-distills with the water in the sample. The condensed solvent and water are continuously separated in a trap wherein the water settles in the graduated section of the trap, and the solvent returns to the distillation flask. The amount of water can then be determined on a volume or a mass basis.
The precision of this method can be affected by water droplets adhering to surfaces in the apparatus and, therefore, not settling into the water trap to be measured. To minimize this problem, all apparatus must be chemically cleaned at least daily to remove surface films and debris that hinder the free drainage of water in the apparatus.
If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises,—the azeotropic composition limits the separation, and for a better separation, this azeotrope must be bypassed in some way. At low to moderate pressure, with the assumption of ideal-gas model for the vapor phase, the vapor-liquid phase equilibrium (VLE) of many mixtures can be adequately described by the following Modified Raoult's Law:yiP=xiγiPisat for i=1, . . . , c 
where                yi=mole fraction of component i in vapor phase;        xi=mole fraction of component i in liquid phase;        P=system pressure;        Psat=vapor pressure of component i; and        γi=liquid-phase activity coefficient of component i.        
When γi=1, the mixture is said to be ideal, and the equation simplifies to Raoult's Law. Nonideal mixtures (γi≠1) can exhibit either positive (γi>1) or negative deviations (γi<1) from Raoult's Law. In many highly nonideal mixtures, these deviations become so large that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phase diagrams exhibit a minimum or maximum azeotrope point. In the context of the T-x, y phase diagram, these points are called the minimum boiling azeotrope (where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components). About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapor phase have the same composition, i.e.:xi=yi for i=1, . . . , c  (2)
Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapor phase; and the heterogeneous azeotropes, where the overall liquid composition, which forms two liquid phases, is identical to the vapor composition. Most methods of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen chemicals to facilitate the separation.
The drawbacks to this method include, for example, the fact that it utilizes hazardous solvents and produces hazardous vapors. This method also takes 2 to 3 hours to complete, and as with most distillation techniques, the accuracy and precision of the results will depend upon the skill of the technician performing the distillation. This method also does not provide any information with regard to the amount of sedimentation in the sample.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.2—Standard Test Method for Water in Crude Oil Distillation for the complete protocol, which is hereby incorporated by reference.)
Superabsorbent Polymers
Superabsorbent polymers are materials that have the ability to absorb and retain large volumes of water and aqueous solutions. This makes them ideal for use in water absorbing applications such as baby nappies, absorbent medical dressings, and controlled release mediums.
Early superabsorbents were made from chemically modified starch and cellulose and other polymers like poly(vinyl alcohol) PVA, poly(ethylene oxide) PEO, all of which are hydrophilic and have a high affinity for water. When lightly cross-linked, chemically or physically, these polymers became water-swellable but not water-soluble.
Today's superabsorbent polymers are made from partially neutralized, lightly cross-linked poly(acrylic acid), which has been proven to give the best performance versus cost ratio. The polymers are manufactured at low solids levels for both quality and economic reasons, and are dried and milled in to granular white solids. In water, they swell to a rubbery gel that in some cases can be up to 99% water by weight.
The driving forces behind a superabsorbent polymer's water absorbency are osmotic pressure and hydrogen bonding. The difference in the sodium ion concentration between the inside of the polymer and the solution in which it is immersed causes the water to flow in rapidly, trying to balance the number of ions inside and outside the polymer. Also, the polymer chains are lined with carboxyl groups (—COOH) with about 50 to 70% of these in the sodium salt form. In contact with water, the carboxyl groups dissociate into negatively charged carboxylate ions (—COO—). These form hydrogen bonds with water molecules.
In addition, these carboxylate sites repel each other. This repulsion widens the polymer network, thus allowing more water to be absorbed. However, the crosslinked polymer molecules can only stretch so far. Therefore, the amount of water absorbed is in part determined by the balance between widening of the polymer network as hydrogen bonds form and “elastic pressure” of the crosslinked molecules.
The electrolyte concentration in the water being absorbed greatly affects the amount of fluid that can be absorbed by the polymer. The ions of the electrolyte act as “contaminator” ions and become positioned along the polymer chain at the carboxylate sites. The electrolyte ions partially neutralize these sites and limit the potential sites for hydrogen bond formation between the polymer and water molecules. In addition, there will be less repulsion between the crosslinked chains of the polymer, thus enabling less water to be absorbed. Also, the presence of these ions decreases the unbalance that “drives” osmotic pressure.
A typical superabsorbent polymer may absorb about 400 times its own mass of distilled water while absorbing almost 300 times its mass of tap water. The same polymer may absorb only 30 to 40 times its mass of 1% sodium chloride solution.
Volume-Differential Assay Using Hydrophilic Gel
The present inventions describe systems and methods for the determination of the water and sediment content in a petroleum sample.
The present innovations include, in one embodiment, collecting a sample to be tested in a field bottle. The sample from the field bottle is then transferred from the field bottle, and into and through a measurement column containing a high-uptake gelling agent, such as a superabsorbent polymer. The sample's moisture can be determined by measuring the expanded volume of the high-uptake gelling agent inside the measurement column. The measurement column also contains a sediment measurement section having a sediment filter and a sight glass with graduations to measure the sediment content of the sample.
Hence, the disclosed innovations provide a simple approach to measuring the moisture and sediment content in crude oil that is extremely fast, accurate, and reproducible without the use of hazardous chemicals. Other embodiments are described more fully below.